Plants are planted in commercially operated plant systems, especially in greenhouses, in natural substrates such as soils, but increasingly in synthetic substrates such as, e.g., rock wool. The cultures are irrigated with water containing nutrients. Water running through the substrate and not absorbed by the plants, which is referred to herein as drainage water, is reused for irrigation.
Various microorganisms accumulate in closed water systems, which microorganisms also include phytopathogenic microorganisms, which cause the plants to die off or to be reduced in quality. Plants such as lettuces, cucumbers and chicory, and also decorative plants, are attacked to a particularly great degree by "fusarium" organisms. It has not been possible previously to entirely remove fusaria from a greenhouse area in a satisfactory manner, e.g. with the known UV treatment or thermal processes, without damaging the plants at the same time. Known processes may also be uneconomical.
Solutions containing peroxycarboxylic acid and/or hydrogen peroxide have been tested in agricultural settings in order to avoid the presence of phytotoxic microorganisms such as fungi, viruses, bacteria, yeasts and algae. According to patent DD 239 109, hydrogen peroxide is added to the irrigation water. However, the action of hydrogen peroxide for combating pathogenic organisms, especially fungi, is unsatisfactory. Moreover, the great amount of water required for use results in significant damage to the roots of the plants.
According to WO 94/20424, better results can be obtained if a solution containing performic acid is added to the drainage water or to the water used for watering. It is preferable to use a solution produced in situ from formic acid and hydrogen peroxide and containing performic acid and hydrogen peroxide. The amount required is in the range of 1 to 1000 ppm performic acid and 25 to 2500 ppm hydrogen peroxide. This method has the disadvantage that the concentrations of performic acid and hydrogen peroxide required for combating fusaria are still so great that damage to the plant roots can readily occur. The root damage due to too great a concentration of peroxide compounds can exceed the damage caused by an infection with fusarium. In general, only a concentration of around or below 30 ppm hydrogen peroxide is viewed as harmless.
The invention had the problem of providing an improved method of combating phytopathogenic microorganisms, especially fungi such as fusaria, in plant systems with an irrigation system, by which method damage to the plants is avoided.